Light-Emitter-Based Devices with Lattice-Mismatched Semiconductor Structures

ABSTRACT

Some aspects for the invention include a method and a structure including a light-emitting device disposed over a second crystalline semiconductor material formed over a semiconductor substrate comprising a first crystalline material.

RELATED APPLICATION

This application claims priority to and the benefits of U.S. Provisional Patent Application Ser. No. 60/852,781, filed on Oct. 19, 2006, the entire disclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to semiconductor processing, and particularly to formation of light-emitting devices based on lattice-mismatched semiconductor structures.

BACKGROUND

Achieving epitaxial growth and fabrication of high-performance optoelectronic devices (i.e., III-V devices) on silicon substrates has been a goal of the semiconductor industry for decades. A major challenge has been to overcome the large lattice-mismatch and thermal-expansion differences between these two fundamentally different material systems. Various methods have been used in the past to demonstrate III-V-based lasers on Si substrates, for example utilizing very thick (˜10 micrometer (μm)) epitaxial buffer layers (see “Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers,” Michael E. Groenert, Christopher W. Leitz, Arthur J. Pitera, and Vicky Yang, Journal of Applied Physics 93 362 (2003)) or utilizing wafer bonding between a Si wafer and epitaxial layers grown on a III-V substrate (see “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Alexander W. Fang, Hyundai Park, Oded Cohen, Richard Jones, Mario J. Paniccia, and John E. Bowers, Optics Express, Vol. 14, Issue 20, pp. 9203-9210 (2006)). However, these methods may have disadvantages from either an economic or a technical standpoint. It may be desirable to avoid both (a) thick epitaxial layers that may be time-consuming to grow, and may suffer from thermal mismatch problems, and (b) bonding between Si and III-V wafers that can suffer from poor yield, thermal mismatch problems, and a lack of commercially available III-V substrates compatible with the Si substrates used today in leading-edge manufacturing. Particularly desirable is an epitaxial solution that can (a) achieve substantial elimination of defects arising from lattice mismatch with no more than about 1 μm of growth, and (b) manage large degrees of thermal mismatch (i.e., mismatch between the thermal expansion coefficients of the substrate and the epitaxial layers).

SUMMARY

Aspect ratio trapping (ART) technology enables the formation of high-quality semiconductor materials on lattice-mismatched substrates. By properly choosing mask configurations, crystal orientations, and epitaxial growth conditions, various material defects may be trapped in a small localized area within a relatively thin epitaxial layer. Furthermore, because the ART regions are relatively small in area, thermal mismatch problems are substantially reduced, because ART islands can expand and contract elastically (producing no defects) with changes in processing temperature, instead of plastically (producing defects). ART technology greatly increases the quality of lattice-mismatched materials that may be achieved, e.g., with the growth of III-V materials on Si substrates. The straightforward process enables reliable and reproducible results.

Embodiments of the invention include the formation of a GaAs-based edge emission laser on a silicon substrate.

An aspect of the invention features a structure having, in various embodiments, a trench defined in a dielectric layer disposed over a substrate comprising a first crystalline semiconductor material. A second crystalline semiconductor material is disposed in the trench, the second crystalline semiconductor material being lattice-mismatched to the first crystalline semiconductor material. A light-emitting device is disposed over and in contact with at least a portion of the second crystalline semiconductor material.

The trench may have a height h and a width w, and a ratio of h/w≧1. The light-emitting device may be a laser or a light-emitting diode, either of which may include GaAs, AlAs, and/or AlGaAs layers. In some embodiments, the laser or light-emitting diode may include InP, GaP, AlP, InAs, AlAs, GaAs, InSb, AlSb, GaSb, InN, and/or their ternary or quaternary compounds.

The first crystalline semiconductor material may include or consist essentially of at least one of a group IV element or compound, a II-VI compound, or a III-V compound. The second crystalline material may include a II-VI compound and/or a III-V compound.

A top contact may be disposed over at least a portion of the light-emitting device and a second contact may be disposed over and in contact with a portion of the substrate. A plurality of trenches may be formed in the dielectric layer, with the second crystalline semiconductor material disposed in each trench. The distance between two adjacent trenches may be at least equal to an average width of the adjacent trenches.

The light-emitting device may include a laser or a light-emitting diode (LED) epitaxial structure disposed in the trench. The second crystalline semiconductor material may extend above the dielectric layer, and the light-emitting device may comprise a laser or an LED epitaxial structure disposed above a top surface of the dielectric layer.

In another aspect, the invention features a method for forming a structure. Embodiments of the method include forming a trench having a dielectric sidewall and a bottom surface adjacent a substrate comprising a first crystalline semiconductor material. At least a portion of the trench may be filled with a second crystalline semiconductor material lattice-mismatched to the first crystalline semiconductor material. A light-emitting device may be defined over and in contact with at least a portion of the second crystalline semiconductor material.

The trench may have a height h and a width w, and a ratio of h/w≧1. The light-emitting device may be a laser or a light-emitting diode. The trench may be filled by selective epitaxy. In an embodiment, the step for forming a trench further includes defining additional trenches, the step for filling the trench further includes forming a coalesced layer of the second crystalline semiconductor above the trenches, and the step of defining a light-emitting device forms the device over the trenches. In another embodiment, the active region of the light-emitting device may be formed within the trench.

In another aspect, the invention features a structure having a dielectric sidewall disposed proximate a top surface of a substrate comprising a first crystalline material. A second crystalline semiconductor material is disposed proximate the dielectric sidewall, the second crystalline semiconductor material being lattice-mismatched to the first crystalline semiconductor material. A cascade superlattice may be disposed over and in contact with at least a portion of the second crystalline semiconductor material.

In another aspect, the invention features a method for forming a trench-based light-emitting device. Embodiments of the method include forming a trench adjacent a substrate, the trench (i) having a non-crystalline sidewall, (ii) having a width w, (iii) having a height h above a bottom surface of the trench, and (iv) having a ratio of h/w≧1, the bottom surface of the trench including an exposed surface of the substrate. A crystalline semiconductor material may be epitaxially grown in at least a portion of the trench, the crystalline semiconductor material being lattice-mismatched to the substrate. A light-emitting device may be formed over the crystalline semiconductor material, an active region of the light-emitting device being at least partially disposed within the trench.

In another aspect, the invention features a method for forming a light-emitting device using lattice-mismatched materials and planarization. Embodiments of the method include forming an opening having a non-crystalline sidewall disposed above a surface of a substrate, the substrate comprising a first crystalline semiconductor material. A second crystalline semiconductor material may be formed in the opening, the second crystalline semiconductor material being lattice-mismatched to the first crystalline semiconductor material. A surface of the second crystalline semiconductor material may be planarized. A light-emitting device may be formed at least partially above the planarized surface of the second crystalline semiconductor material.

BRIEF DESCRIPTION OF FIGURES

In the drawings, like reference characters generally refer to the same features throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIGS. 1-5 are schematic cross-sectional views illustrating a method for formation of a device on a semiconductor substrate in accordance with an aspect of the invention;

FIG. 6 is a simplified schematic perspective view of a device formed in accordance with the method illustrated in FIGS. 1-5;

FIGS. 7 a-7 d are schematic cross-sectional views illustrating alternative structures formed in accordance with aspects of the invention;

FIGS. 8 a-8 c are schematic cross-sectional views illustrating a method for formation of a V-groove laser diode in accordance with an aspect of the invention; and

FIGS. 9 a-9 b are schematic cross-sectional views illustrating a method for formation of III-V cascade lasers in accordance with an aspect of the invention.

FIG. 10 is a plan view illustrating examples of photonic patterning.

FIGS. 11 a-b are plan views illustrating one-dimensional laser diode and light-emitting diode arrays.

DETAILED DESCRIPTION

Referring to FIG. 1, a substrate 100 includes a first crystalline semiconductor material S1. The substrate 100 may be, for example, a bulk silicon wafer, a bulk germanium wafer, a semiconductor-on-insulator (SOI) substrate, or a strained semiconductor-on-insulator (SSOI) substrate. The substrate 100 may include or consist essentially of the first semiconductor material S1, such as a group IV element, e.g., germanium or silicon. In an embodiment, substrate 100 includes or consists essentially of n-type (100) silicon.

A dielectric layer 110 is formed over the semiconductor substrate 100. The dielectric layer 110 may include or consist essentially of a dielectric material, such as silicon nitride or silicon dioxide. The dielectric layer 110 may be formed by any suitable technique, e.g., thermal oxidation or plasma-enhanced chemical vapor deposition (PECVD). As discussed below, the dielectric layer may have a thickness t₁ corresponding to a desired height h of crystalline material to be deposited in a trench formed through the dielectric layer. In some embodiments, the thickness t₁ of the dielectric layer 110 is in the range of 25-1000 nm. In a preferred embodiment, the thickness t₁ is 500 nm.

A mask (not shown), such as a photoresist mask, is formed over the substrate 100 and the dielectric layer 110. The mask is patterned to expose at least a portion of the dielectric layer 110. The exposed portion of the dielectric layer 110 is removed by, e.g., reactive ion etching (RIE) to define a trench 120. Trench 120 may be defined by at least one sidewall 130, and may extend to a top surface 135 of the substrate 100. The height h of the sidewall 130 corresponds to the thickness t₁ of the dielectric layer 110, and may be at least equal to a predetermined distance H from a top surface 135 of the substrate. The predetermined vertical distance H is a minimum distance at which a majority of defects in a lattice-mismatched material formed in the trench terminate at the sidewall 130.

The trench may be substantially rectangular in terms of cross-sectional profile, a plan view, or both, and have a width w that is smaller than a length L (not shown) of the trench. For example, the width w of the trench may be less than about 500 nm, e.g., about 10-100 nm, and the length L of the trench may exceed each of w and H. A ratio of the height h of the trench to the width w of the trench 120 may be ≧0.5, e.g., ≧1.

A second crystalline semiconductor material S2, i.e., crystalline material 140, is formed in the trench 120. The crystalline material 140 may include or consist essentially of a group IV element or compound, a III-V compound, or a II-VI compound. Examples of suitable group IV elements or compounds include germanium, silicon germanium, and silicon carbide. Examples of suitable III-V compounds include gallium antimonide, gallium arsenide, gallium nitride, gallium phosphide, aluminum antimonide, aluminum arsenide, aluminum nitride, aluminum phosphide, indium antimonide, indium arsenide, indium nitride, indium phosphide, and their ternary or quaternary compounds. Examples of suitable II-VI compounds include zinc selenide, zinc sulfide, cadmium selenide, cadmium sulfide, and their ternary or quaternary compounds.

The crystalline material 140 may be formed by selective epitaxial growth in any suitable epitaxial deposition system, including, but not limited to, metal-organic chemical vapor deposition (MOCVD), atmospheric-pressure CVD (APCVD), low- (or reduced-) pressure CVD (LPCVD), ultra-high-vacuum CVD (UHVCVD), molecular beam epitaxy (MBE), or atomic layer deposition (ALD). In the CVD process, selective epitaxial growth typically includes introducing a source gas into the chamber. The source gas may include at least one precursor gas and a carrier gas, such as, for example, hydrogen. The reactor chamber may be heated by, for example, RF-heating. The growth temperature in the chamber may range from about 300° C. to about 900° C., depending on the composition of the crystalline material. The growth system may also utilize low-energy plasma to enhance the layer growth kinetics.

The epitaxial growth system may be a single-wafer or multiple-wafer batch reactor. Suitable CVD systems commonly used for volume epitaxy in manufacturing applications include, for example, an Aixtron 2600 multi-wafer system available from Aixtron, based in Aachen, Germany; an EPI CENTURA single-wafer multi-chamber systems available from Applied Materials of Santa Clara, Calif.; or an EPSILON single-wafer epitaxial reactor available from ASM International based in Bilthoven, The Netherlands.

In an exemplary process, a two-step growth technique is used to form high-quality crystalline material 140, consisting essentially of GaAs, in the trench 120. First, the substrate 100 and dielectric layer 110 are thermally annealed with hydrogen at approximately 800° C. for approximately 15 minutes to desorb a thin volatile oxide from the substrate surface 135 that may be produced during pre-epitaxy wafer preparation. Chamber pressure during annealing may be in the range of approximately 50-100 torr, for example 75 torr. After annealing, the chamber temperature is cooled down with hydrogen flow. In order to suppress anti-phase boundaries (APBs) on substrate surface 135, a pre-exposure to As for about 1 to 2 minutes is performed. This step helps ensure uniform coverage of the trench surface with an As—As monolayer. This pre-exposure is achieved by flowing AsH₃ gas through the reactor at a temperature of approximately 460° C. Then, the precursor triethylgallium (TEG) or trimethylgallium (TMG) is introduced into the chamber together with AsH₃ gas at a higher growth temperature, e.g., approximately 500° C. to 550° C. promote the initial GaAs nucleation process on the As pre-layer surface. This high-temperature process helps ensure that the Ga atoms are sufficiently mobile to avoid GaAs cluster formation. A slow growth rate of about 2 to 4 nm per minute with V/II ratio of about 50 may be used to obtain this initial GaAs layer, with a thickness in the range of about 50 to 100 nm.

Then a layer of n-type GaAs having a thickness of 1 to 2 μm is grown at a constant growth temperature of approximately 680° C. and a V/III ratio of approximately 80 to obtain defect-free GaAs material inside the trench 120. During this step, the GaAs epitaxial layer thickness t₂ may be greater than the dielectric mask thickness t₁. The GaAs material may have a mushroom-type cross-sectional profile with lateral over growth over the dielectric layer 110; the top portion of the GaAs material may coalesce with GaAs formed in neighboring trenches (not shown) to form an epitaxial layer. The width w₂ of the crystalline material 140 extending over a top surface 160 of the dielectric layer 110 may be greater than the width w of the trench 120. In this case, a small void may be formed between the laterally grown GaAs layer and the top surface 160 of the dielectric layer 110. The overall layer thickness t₂ of the crystalline material 140 may be monitored by using pre-calibrated growth rates and in situ monitoring equipment, according to methods routinely employed in the art.

Most, if not all, dislocation defects 150 in the crystalline material 140 reach and terminate at the sidewalls of the trench 120 in the dielectric material 110 at or below the predetermined vertical distance H from the surface 135 of the substrate, such that dislocations in the crystalline material 140 decrease in density with increasing distance from the bottom portion of the trench 140. Accordingly, the upper portion of the crystalline material is substantially exhausted of dislocation defects. Various dislocation defects such as threading dislocations, stacking faults, twin boundaries, or anti-phase boundaries may thus be substantially eliminated from the upper portion of the crystalline material.

The crystalline material 140 may be considered to have two portions: a lower portion for trapping dislocation defects and an upper portion which either (a) incorporates the laser or LED epitaxial layers or (b) serves as a template for the subsequent epitaxial growth of the laser or LED epitaxial layers. The height h of the crystalline material thus has two components: the height h_(trapping) of the lower portion (where defects are concentrated) and the height h_(upper) of the upper portion (which is largely free of defects). The height h_(trapping) of the trapping portion may be selected from a range of about ½w≦h_(trapping)≦2 w, to ensure effective trapping of dislocation defects. The actual value of h_(trapping) required may depend upon the type of dislocation defects encountered, which may depend on the materials used, and also upon the orientation of the trench sidewalls. In some instances, the height h_(trapping) can be greater than that required for effective defect trapping, in order to ensure that the dislocation defects are trapped at a sufficient distance away from the upper portion, so that deleterious effects of dislocation defects upon device performance are not experienced. For example, h_(trapping) may be, e.g., 10-100 nm greater than required for effective trapping of defects. For the upper portion, the height h_(upper) may be selected from the range of approximately ½w≦h_(upper)≦10w.

It has been observed experimentally that dislocations in a mismatched cubic semiconductor grown on a Si (100) surface in the near vicinity (e.g., within approximately 500 nm or less) of a vertical dielectric sidewall surface bend toward that surface at approximately 30 degrees through 60 degrees. For example, the dislocations may bend toward that surface at approximately a 45-degree angle to that surface. Based on this relationship, the predetermined distance H necessary to trap defects is, typically, approximately equal to a width between ½w and 2w, where w is the width of the trench. This range is based on the range of intersection angles of approximately 30 degrees through 60 degrees; then, tan(30°)w≦H≦tan(60°)w, which roughly corresponds to ½w≦H≦2w.

Referring to FIG. 2, a planarization step such as, e.g., chemical-mechanical polishing (CMP) may be used to planarize a top surface 170 of the crystalline material 140 within a roughness level preferably no greater than 20 nm, to allow the formation of good quality films thereon. In an embodiment, the thickness t₃ of the portion of the crystalline material 140 disposed within the trench 120 is 1 μm and the thickness 4 of the portion of the crystalline material 140 disposed above the top surface 160 of the dielectric layer 110 is about 0.5 μm. In another embodiment, a planarization step such as, e.g., CMP is used to planarize a top surface of the crystalline material 140 within a roughness level preferably no greater than 20 nm, to ensure that the top surface of the crystalline material 140 is substantially co-planar with the top surface 160 of the dielectric material 110.

Referring to FIG. 3, a laser epitaxial structure 300 may be formed over the crystalline material 140. For example, the laser epitaxial structure 300 may be a GaAs-based laser structure including layers of GaAs/InGaAs/AlGaAs, formed according to methods known in the art. The laser structure, described from the bottom up, may include, e.g., a ˜1.5 μm thick n-GaAs base layer 310, a ˜0.30 μm n-Al_(0-0.4)GaAs graded lower cladding layer 312, a ˜1.3 μm thick n-Al_(0.4)GaAs cladding layer 314, an undoped ˜0.3 μm Al_(0.4-0.2)GaAs graded lower confining layer 316, an undoped ˜7 nm In_(0.2)GaAs single quantum well layer 318, an undoped ˜0.3 μm Al_(0.2-0.4)GaAs graded upper confining layer 320, a ˜1.3 μm thick p-Al_(0.4)GaAs cladding layer 322, a ˜0.30 μm thick p-Al_(0.4-0)GaAs graded upper cladding 324, and a ˜0.1 μm thick p⁺-GaAs contact layer 326. These layers may be formed by epitaxial growth in any suitable epitaxial deposition system, including, but not limited to, MOCVD, APCVD, LPCVD, UHVCVD, MBE, or ALD. A growth temperature may be approximately 550˜680° C. for the GaAs layer and approximately 650˜800° C. for the AlGaAs layers. In an embodiment, GaAs-based laser epitaxial structures are grown on offcut Si substrates oriented about 2° off (100) towards the <011> direction in an AIXTRON 2600 MOCVD reactor at about 75 torr. TMG and trimethylaluminum (TMA) may be used as group-III sources, and arsine (AsH₃) may be used as the group-V source. Silane (SiH₄) and carbon tetrachloride (CCl₄) may be used as n- and p-dopant sources, respectively. Hydrogen may be used as a carrier gas.

Referring to FIG. 4, instead of a laser epitaxial structure 300, an LED epitaxial structure 300′ may be formed over the crystalline material 140. The surface-emitting epitaxial LED structure includes a base layer 310′, a lower cladding layer 314′, an undoped lower confining layer 316′, an undoped single- or multiple-quantum-well active region 318′, an undoped upper confining layer 320′, an upper cladding layer 322′, and a heavily doped p-type contact layer 326′. Metal contacts may be formed over the p-type contact layer 326′ using the process described below with reference to FIG. 5. It is often desired that light emission for LEDs be through the top surface. For this reason, a transparent top-contact material such as indium tin oxide (ITO) may be used. Alternatively, a non-transparent contact may be used, preferably not covering an entire top surface of the LED epitaxial structure.

Referring to FIG. 5, an RIE etch step may be performed to remove a portion of the laser epitaxial structure 300 that may extend laterally over the dielectric layer 110. This etch may remove defects that may arise from coalescence between neighboring selective epitaxial regions. Referring also to FIG. 6, at the same time, this etch may define the front and back facets 600, 610 of an edge-emission laser structure 620. The etch may be performed with, for example, argon and chlorine plasmas. A metal layer is deposited over the dielectric layer, for example via evaporation, and patterned to define a top contact 520. The metal layer may have a thickness of, e.g., approximately 150 nm. In an embodiment, the top contact 520 is a p-type contact and includes or consists essentially of Ti/Pt/Au. Following contact deposition, an anneal of approximately 350-450° C. for approximately 10 minutes may be performed to ensure good electrical contact with the laser epitaxial structure 300. A second metal contact 530, e.g., a bottom metal contact, is defined adjacent the substrate 100. A backside of the substrate 100 may be thinned before the second metal contact is defined. In an embodiment, the second metal contact is formed on the backside of the substrate. In an alternative embodiment, the second metal contact contacts the topside of the substrate through a window defined in the overlying dielectric layer(s) 110 (see FIGS. 7 a-7 d). The second metal contact may have a thickness of, e.g., approximately 500 nm. In some embodiments, the second metal contact is an n-type contact and includes or consists essentially of Al formed by evaporation on an n-type Si substrate. A standard laser facet coating process is used, according to methods known in the art, to obtain reflectivity of approximately 3% and approximately 95% for front and back facets, respectively. The resulting structure is a completed laser device that can be integrated into circuits formed on a semiconductor die. The laser device is a light-emitting device 620 including laser epitaxial structure 300.

A GaAs-based laser structure growth and device preparation process are illustrated in FIGS. 1-6. The figures are not to scale, and a thick laser structure is illustrated to demonstrate the general process sequence. This laser structure includes a single InGaAs quantum well centered in a waveguide, graded-index separate confinement heterostructure (GRINSCH) with 1.3-μm-thick Al_(0.4)Ga_(0.6)As cladding layers. The illustrated device is designed for operation at a wavelength of 980 nm. However, the process may be utilized to form other structures based on ART technology. For instance, for small dimensional devices such as nano-scale light emitters, the full epitaxial structure for the laser may be confined inside a single trench. In this case, CMP process and laser mesa contact process may be eliminated or simplified. Also, as discussed below, this method may be used to fabricate broad-area light emitters or emitter arrays, with large device dimensions and covering a plurality of trenches.

Referring to FIGS. 7 a-7 d, the process described above with respect to FIGS. 1-6 may be used to form light-emitting structures having various configurations. Referring to FIG. 7 a, laser or LED epitaxial structure 300 or 300′ may be formed in trench 120. Dislocation defects 150 may be trapped within the lower trench region 710. In an embodiment, a planarization process, for example, CMP, of the crystalline material 140 is not necessary, as growth of the crystalline material 140 is halted before the crystalline material completely fills the trench 120. In this way, sufficient room (e.g., approximately 1-3 μm) is provided in an upper portion of the trench 120 for the formation of the laser or LED epitaxial structure. In some embodiments, the crystalline material 140 defines a portion of the laser or LED epitaxial structure 300 or 300′. In other embodiments, the crystalline material 140 provides a template for the formation of the laser or LED epitaxial structure 300 or 300′ in the trench 120. Top contact 520 is formed over the laser or LED epitaxial structure 300 or 300′. The second metal contact 530 contacts the topside of the substrate 100 through a window defined in the overlying dielectric layer(s) 110 (see FIGS. 7 a-7 d).

Referring to FIG. 7 b, as discussed in detail with respect to FIGS. 1-6, laser or LED epitaxial structure 300 or 300′ may be formed on a top portion of the crystalline material 140 extending above the top surface 160 of the dielectric layer 110.

Referring to FIG. 7 c, a plurality of trenches, i.e., two or more adjacent trenches may each include crystalline material 140 and a laser or LED epitaxial structure 300 or 300′ disposed over the crystalline material 140 in the trench, as discussed with reference to FIG. 7 a. The distance d between two adjacent trenches may be at least equal to the average width of the adjacent trenches. A patterned metal or a transparent top contact 520 may be formed over and in contact with each of the laser or LED epitaxial structures 300 or 300′, thereby providing a broad-area laser or LED.

Referring to FIG. 7 d, two or more adjacent trenches may each include crystalline semiconductor 140 extending above the top surface 160 of the dielectric layer 110, as described above with reference to FIGS. 1-6 and FIG. 7 b. Lateral growth of the crystalline material 140 between the adjacent trenches 120 results in the coalescence of the top portions of the crystalline material into a single epitaxial film 700 of crystalline material 140. A planarization process, for example, CMP, may be performed on a top surface of the epitaxial film 700. Thereafter, a broad area laser or LED epitaxial structure 300 or 300′, containing an optical active region, is formed over the epitaxial film 700.

Referring to FIGS. 8 a-8 c, the process described above with reference to FIGS. 1-6 may be combined with selective etching to form a V-groove laser diode. Referring to FIG. 8 a, in an exemplary embodiment, crystalline semiconductor material 140 is formed in a trench 120, and extends beyond the top surface 160 of dielectric 110. In an embodiment, an electrically insulating diode (EID) (not shown) is added. The EID may be formed from two layers of semiconductor material, and the crystalline semiconductor material 140 may be the first of those layers. The EID may be positioned between the crystalline material 140 formed in trench 120 and the light-emitting device 300 or 300′. An n-contact 530 may be used to make an electrical connection to the crystalline material 140 above the trench 120, for example, near the top surface 160 of the dielectric layer 120; an EID may preferably then be used to prevent leakage current from the substrate 100 into the trench 120. The crystalline material 140 is preferably planarized by, for example, CMP to within a roughness level preferably no greater than 20 nm.

Referring to FIG. 8 b, the top surface of the crystalline semiconductor material 140 may then be modified with a selective etching process to create a V groove 800 in the surface of the crystalline material 140. In particular, the V groove may be formed as follows. An upper dielectric layer 810 including a dielectric material, e.g., Si₃N₄, may be formed on the crystalline semiconductor material 140 using a deposition process, e.g., PECVD, and patterned by photolithography and etching. The pattern in the upper dielectric layer 910 may include stripes oriented along the trench 120 and may have dimensions similar to those of the lower dielectric layer 110. In an embodiment, the stripes defined by the upper dielectric layer have dimensions greater than those of the lower dielectric layer 110, so that the upper dielectric layer 140 covers some trenches 120 but not other trenches 120. The pattern in the upper dielectric layer may be defined by a wet etch, e.g., a buffer oxide etchant.

An appropriate wet etch, e.g., H₂SO₄:H₂O₂:H₂O=1:8:4, may then be used to create a V-groove in the crystalline semiconductor material 140 exposed by the openings formed in the upper dielectric layer. The appropriate wet etch etches the crystalline semiconductor material 140 selectively with respect to the upper dielectric layer and dielectric layer 110. Moreover, the wet-etch composition is selected to define facets in the crystalline semiconductor material 140 to form the V-groove. Subsequently, the patterned upper dielectric layer may be removed.

Referring to FIG. 8 c, a diode structure 300 is grown on top of the V-shaped layer to create a V-groove laser diode. This design may be applied to the fabrication of single or trenched array laser diodes.

Referring to FIGS. 9 a-9 b, the process described above with reference to FIGS. 1-6 may be combined with the formation of cascade superlattice structures to create cascade lasers. Referring to FIG. 9 a, a multiple stage cascade structure 900 may be formed with a preferred structure design and controlled layer thicknesses, and may include, for example, InP/GaInAs/AlInAs, GaAs/AlGaAs or Si/SiGe/Si superlattice layers. The cascade structure 900 may be formed within trench 120 above dislocation defects 150. Each layer forming the cascade structure 900 may range in thickness from, for example, 1 nm to 5 nm, and may be formed with an epitaxial process, for example, MBE or MOCVD. The layers of the superlattice 900 may be formed in groups of, for example, 5-10 layers, where the composition and number of the layers within a group defines the output characteristics of the laser. Furthermore, the group of layers may be repeated, for example, 10-30 times, to increase the output power of the laser. The number of layers within a group and number of groups may be repeated as many times as the height of the crystalline semiconductor material 140 allows. Growth of the cascade structure 900 may terminated before coalesced growth occurs, to produce a near- or middle-infrared laser with peak emission at a wavelength between 3 μm and 17 μm. In some embodiments, the crystalline material 140 itself provides a template for the formation of the cascade superlattice structure 900 above the trench 120.

In the illustrated embodiment, the top contact 520 is formed over the cascade superlattice structure 900. The second metal contact 530 contacts the bottom side of the thinned substrate 100.

Referring to FIG. 9 b, multiple-wavelength cascade lasers may be formed on a surface that is preferably planarized, e.g., by CMP to within a roughness level preferably no greater than 20 nm. Different wavelength emissions may be accomplished by multiple sectional etching processes. The emission wavelength is tuned over a wide range by applying a voltage using multiple-section contacts. In an embodiment, one or all of the multiple wavelength sections of the cascade superlattice structure 900 has its own n-contact 530.

Referring to FIG. 10, the process described above with respect to FIGS. 1-7 may be modified to create photonic crystals. The dielectric layer 110 may be first deposited, and then etched to produce trenches and shapes other than trenches, e.g., circular holes. These shapes correspond to features in crystalline material 140 that may be formed to direct emissions from a laser. A photonic mask layer (not shown) may be created to define these shapes and the dielectric layer 110 is selectively removed, e.g., etched, to form the desired shapes. After the dielectric layer 110 has been patterned, openings in the patterned dielectric layer 110 are filled with the crystalline semiconductor material 140. In an embodiment, an EID as described above is formed. The dielectric layer may then be wholly or partially removed to form an air-spaced photonic crystal array, i.e., having features separated by air. In an embodiment, part of the dielectric layer is left in place to change the refraction index of the boundary between the crystalline semiconductor material 140 and the dielectric layer 110 in the photonic crystal array. In various embodiments, this crystal array is used with a laser 1010 as an optical DBR reflector 1020, beam divider 1030, or waveguide 1040, as illustrated in FIG. 10.

Referring to FIGS. 11 a-b, the method described in FIGS. 1-7 may be modified to form a number of light-emitting devices arranged in a one-dimensional array. A number of trenches of arbitrary length may be placed in close proximity to each other, and each trench may contain a light-emitting device, such as a laser diode or a light-emitting diode. Referring to FIG. 11 a, a plan view of a one-dimensional array of light-emitting diodes is shown. The p-contact 520 may cover only part of the top surface of the diode structure 300′ to permit the diode to emit light perpendicular to the page from the portion of the top surface of the diode structure 300′ not covered by p-contact 520. Each diode in the array may be electrically connected by forming a layer of metal 1110 at one end of the trenches 120 that is electrically connected to each diode's p-contact 520. Neighboring diodes may be electrically isolated with a dielectric isolator structure 1130. Referring to FIG. 11 b, a plan view of a one-dimensional array of laser diodes is shown. The p-contact 520 may cover the entire top surface of the laser structure 300, and the laser may output light in a direction 1120, within the plane of the page. Each laser in the one-dimensional array may be electrically connected by forming a layer of metal 1110 at one end of the trenches 120 that is electrically connected to each laser's p-contact 520. Neighboring lasers may be electrically isolated with a dielectric isolator structure 1130.

Each light-emitting device in a one-dimensional array may be formed inside its own trench 120, as shown in FIG. 7 a. In this case, each trench-defined device may be electrically isolated from a neighboring device or devices by portions of the dielectric layer 110.

In another embodiment, the light-emitting devices in the one-dimensional array are formed above the trench 120, as shown in FIG. 7 b, in the single epitaxial layer resulting from the crystalline semiconductor material 140 coalescing above and between neighboring trenches. In this case, an etch step may be performed to electrically isolate the devices. This process may include the steps of (i) forming a masking layer over the light-emitting devices, (ii) removing material from the masking layer to expose portions of the crystalline semiconductor material 140 where electrical continuity is to be broken, for example, the midpoint between two trench-based light-emitting devices, and (iii) removing the exposed crystalline semiconductor material 140 with an etching process. The gaps formed when the exposed crystalline semiconductor material 140 is removed may be filled with, for example, a dielectric material 1130.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 

1. A structure comprising: a trench defined in a dielectric layer disposed over a substrate comprising a first crystalline semiconductor material; a second crystalline semiconductor material disposed in the trench, the second crystalline semiconductor material being lattice-mismatched to the first crystalline semiconductor material; and a light-emitting device disposed over and in contact with at least a portion of the second crystalline semiconductor material.
 2. The structure of claim 1, wherein the trench has a height h and a width w, and a ratio of h/w≧1.
 3. The structure of claim 1, wherein the light-emitting device is a laser or a light-emitting diode.
 4. The structure of claim 3, wherein the light-emitting device is a laser and comprises at least one of a GaAs, an AlAs, or an AlGaAs layer.
 5. The structure of claim 3, wherein the light-emitting device is a laser and comprises at least one of InP, GaP, AlP, InAs, AlAs, GaAs, InSb, AlSb, GaSb, InN, or their ternary or quaternary compounds.
 6. The structure of claim 3, wherein the light-emitting device is a light-emitting diode and comprises a GaAs, an AlAs, or an AlGaAs layer.
 7. The structure of claim 3, wherein light-emitting device is a light-emitting diode and comprises InP, InAs, AlAs, GaAs, AlSb, GaSb, or their ternary or quaternary compounds.
 8. The structure of claim 1, wherein the first crystalline semiconductor material comprises at least one of a group IV element or compound, a II-VI compound, or a III-V compound.
 9. The structure of claim 1, wherein the second crystalline material comprises at least one of a II-VI compound or a III-V compound.
 10. The structure of claim 1, further comprising a top contact disposed over at least a portion of the light-emitting device and a second contact disposed over and in contact with a portion of the substrate.
 11. The structure of claim 1, wherein a plurality of trenches are formed in the dielectric layer, the second crystalline semiconductor material being disposed in each trench.
 12. The structure of claim 11, wherein a distance between two adjacent trenches is at least equal to an average width of the adjacent trenches.
 13. The structure of claim 1, wherein the light-emitting device comprises a laser epitaxial structure disposed in the trench.
 14. The structure of claim 1, wherein the light-emitting device comprises an LED epitaxial structure disposed in the trench.
 15. The structure of claim 1, wherein the second crystalline semiconductor material extends above the dielectric layer, and the light-emitting device comprises a laser epitaxial structure disposed above a top surface of the dielectric layer.
 16. The structure of claim 1, wherein the second crystalline semiconductor material extends above the dielectric layer, and the light-emitting device comprises an LED epitaxial structure disposed above a top surface of the dielectric layer.
 17. A method for forming a structure, the method comprising: forming a trench having a dielectric sidewall and a bottom surface adjacent a substrate comprising a first crystalline semiconductor material; filling at least a portion of the trench with a second crystalline semiconductor material lattice-mismatched to the first crystalline semiconductor material; and defining a light-emitting device over and in contact with at least a portion of the second crystalline semiconductor material.
 18. The method of claim 17, wherein the trench has a height h and a width w, and a ratio of h/w≧1.
 19. The method of claim 17, wherein the light-emitting device is selected from the group consisting of a laser and a light-emitting diode.
 20. The method of claim 17, wherein the trench is filled by selective epitaxy.
 21. The method of claim 17, further comprising the step of defining additional trenches, wherein the step of filling comprises forming a coalesced layer of second crystalline semiconductor material above the trenches, and the step of defining a light-emitting device comprises defining a light-emitting device over the trenches.
 22. The method of claim 17, wherein an active region of the light-emitting device is formed within the trench.
 23. A structure comprising: a dielectric sidewall disposed proximate a top surface of a substrate comprising a first crystalline material; a second crystalline semiconductor material disposed proximate the dielectric sidewall, the second crystalline semiconductor material being lattice-mismatched to the first crystalline semiconductor material; and a cascade superlattice disposed over and in contact with at least a portion of the second crystalline semiconductor material.
 24. A method for forming a trench-based light-emitting device, the method comprising: forming a trench adjacent a substrate, the trench (i) having a non-crystalline sidewall, (ii) having a width w, (iii) having a height h above a bottom surface of the trench, and (iv) having a ratio of h/w≧1, the bottom surface of the trench including an exposed surface of the substrate. epitaxially growing a crystalline semiconductor material in at least a portion of the trench, the crystalline semiconductor material being lattice-mismatched to the substrate; and forming a light-emitting device over the crystalline semiconductor material, an active region of the light-emitting device being at least partially disposed within the trench.
 25. A method for forming a light-emitting device using lattice-mismatched materials and planarization, the method comprising: forming an opening having a non-crystalline sidewall disposed above a surface of a substrate, the substrate comprising a first crystalline semiconductor material; forming a second crystalline semiconductor material in the opening, the second crystalline semiconductor material being lattice-mismatched to the first crystalline semiconductor material; planarizing a surface of the second crystalline semiconductor material; and forming a light-emitting device at least partially above the planarized surface of the second crystalline semiconductor material. 